Polymerizable compositions materials may cure, or harden, when they are subjected to light because a polymerization reaction is photo-initiated. Although these type of compositions have a wide range of applications, they are especially useful in the field of dentistry for adhesion, sealing and restoration. These are known as “photo-curable dental compositions” (see for example, those dental compositions described in U.S. Pat. Nos. 4,553,940, 4,437,836, 4,411,625 and 6,387,980, the entireties of which are incorporated herein by reference).
The quality of the cured compositions is important in all photo-curable polymer applications, e.g., semiconductor applications, but is of special concern in the field of dentistry. Characteristics which are significant in evaluating the quality of a photo-cured polymer include hardness, depth of cure, polymerization yield, and uniformity.
Photo-curable dental compositions generally harden when exposed to radiation having wavelengths in the visible range. Photo-cured dental compositions are convenient for use by a dentist because the curing process can be initiated when the dental composition has been accurately placed in its proper position. A source of radiation energy positioned proximate to the material to be hardened, for example an appropriate amount of composition placed inside a tooth cavity, is activated to initiate polymerization and subsequent curing of the composition to secure the repair. Early methods for curing photosensitive dental compositions included dental guns and other apparatuses for producing concentrated beams of UV radiation. See U.S. Pat. Nos. 4,112,335 and 4,229,658, for example. Later, visible light curable dental compositions were used and dental radiation guns for producing concentrated visible light were provided like that disclosed in U.S. Pat. Nos. 4,385,344 and 6,171,105.
U.S. Pat. No. 4,309,617 to Long discloses an apparatus for providing controlled flashes of radiation, including a hand-piece within which is mounted a gaseous discharge tube and a light pipe for conducting the radiation, enabling it to be directed into an oral cavity to effect in situ curing of radiation polymerizable resins used as dental restoratives. The flash durations, which can be 0.5 milliseconds in duration (See Col. 6 lines 32-34) and intervals are controlled to minimize generation of heat.
There is shown in U.S. Pat. No. 5,420,768 to Kennedy, for instance, a portable photo-curing device that has a light emitting diode matrix energized with battery power. The '768 patent notes in Col. 2 that light emitting diodes (LEDs) of various selected colors may be formed on the module by using selected color dyes so that the emitted light is a pure white light or a combination of selected color lights to provide a predetermined photo curing effect. The light emitted by the LEDs may have a peak wavelength of 470 nm which is used for photo curing purposes.
U.S. Pat. No. 5,634,711 to Kennedy et al. discloses a portable light emitting device suitable for medical and industrial photo curing. Typically, the LEDs are driven by a pulsed power supply in order to minimize heat generation. It is noted in the '711 patent that various applications require different light dosage values. For example, it is noted in Col. 1, lines 39 and following that light dosage values in the range of up to 400 mW/cm2 are typically required for dental applications. On the other hand, a medical application such as photodynamic therapy of psoriasis and basal cells requires much lower power typically in the range of up to 100 mW/cm2. The device according to the '711 patent includes generally a power supply, a housing, and a substrate upon which a plurality of light emitting diodes are mounted. It can be seen from FIGS. 1 and 6 of the '711 patent that the LED array is generally planar and that the device typically includes an optical assembly such as a fiber optic taper. Here again, the LED can comprise “blue” LEDs with a spectral emission in the 470 nanometer range.
U.S. Pat. No. 5,885,082 to Levy discloses the use of laser radiation having a selected wavelength and being in the form of pulses for cutting bone and performing dental procedures. There is disclosed in Column 4, lines 27 and following a filling material for teeth constituted by a mixture formed from a liquid component composed of phosphoric acid and water and a powder component composed of a ceramic and hydroxyapatite, with the ingredients mixed in a proportion to form a paste having a consistency such that the paste is workable and sufficiently self supporting to be applied to the opening with a spatula and remain in place. The '082 patent does not involve a photocuring process and the material is not a dental polymer composite. The high peak power of the laser is believed to be used only for cutting and possibly hardening of the cement due to heat.
U.S. Pat. No. 6,159,005 to Harold et al. discloses an apparatus for photopolymerizing synthetic materials, specifically dental materials containing camphorquinone or phosphine oxide as photoinitiators and includes a light source constituted by a semiconductor base solid state radiation emitter which emits in the blue spectral range. Since the radiation emitter emits in a relatively limited spectral range, excess heat radiation is avoided. The overall device is formed as a relatively small lightweight device with a built in battery. The device further includes a light-conducting rod in order to direct radiation to the desired location. According to the '005, patent an essential photoinitiator in dental materials is typically camphorquinone or phosphine oxide which absorbs a broad band within the blue spectral range, with an absorption maximum of about 472 nm and 430 nm, respectively. The patent further notes that depending on the color of the material, the polymerization reaction requires light having an intensity of at least 1 to 5 mW/cm2 within a very thin layer. In the practice of polymerizing tooth fillings or dental replacement parts, a light intensity of at least 250 mW/cm2 is required within an appropriate period of time to achieve polymerization of sufficient degree and depth. Commercially available dental polymerization apparatuses, at least according to this '005 patent, emit light at an intensity of about 400-500 mW/cm2 sometimes up to 700 mW/cm2. The solid state radiation emitter according to the '005 patent is preferably a laser diode which emits a forward beam used for the polymerization proper and a backward beam used as a reference beam for controlling the intensity of the polymerization beam.
U.S. Pat. No. 6,482,004 to Senn et al. discloses a light curing device and method for curing light-polymerizable dental materials. Such light curing device includes a control circuit which controls the light output of the light source according to a predetermined output profile. The output profile includes an initial curing time wherein the output is increased over a period of time and a main curing time wherein the output is alternated at an alternating high and low output value. Further the output values are subject to change between the initial curing time and the main curing time.
Still further devices and techniques have been proposed as noted below. “Possible improvements of clinical properties of dental composite materials with pulsed blue laser curing” by Zrinka Tarle, Andrej Meniga, Mira Ristic, Jozo Sutalo and Goran Pichler, Croatia Chemica Acta 71, 777-787 (1998) compare the degree of conversion and polymerization shrinkage of hybrid and microfilled composite materials (see Table 1 of paper) cured using a pulsed blue laser system and a “standard” curing unit operating in a continuous wave (cw) mode (Heliolux GTE curing unit of 500 mW/cm2). The goal of the work was to determine whether optimal degree of conversion and minimal polymerization shrinkage of the composite restorative resin could be achieved using nanosecond lasers operating at 468 nm. This wavelength closely matches the maximum of the camphorquinine absorption spectrum. The authors used an excimer pumped dye laser to generate the desired pulsed optical energy necessary for the .photopolymerization. As such, the optical pulses generated although tunable in wavelength, have a fixed temporal characteristics (20 nanosecond laser pulses at a repetition rate of 10 Hz) set by the nature of the lasers used.
“Pulsed blue laser curing of hybrid composite resins” by Andrej Meniga, Zrinka Tarle, Mira Ristic, Jozo Sutalo and Goran Pichler, Biomaterials 18, 1349-1354 (1997) reports on the effect of photopolymerization of light and dark shades cured using 10 mJ of 20 nanosesond laser pulsed dye lasers operating at 468 nm.
U.S. Pat. No. 6,102,696 to Osterwalder et al. discloses a self-contained light source for curing light initiated resins used to coat teeth as veneers and fill cavities and chips in teeth in aesthetic or restorative procedures. The source includes an elongated container holding a battery and an electronic compartment in one end and a light emitting window at the other. A plurality of closely spaced light emitters, typically light emitting diodes or laser diodes are arrayed in a radial or arcuate configuration to direct light to a common focal point. The light is directed out of the container toward a tooth bearing the resin to be cured to a hard stable state. The light emitters are chosen so that they generate light to which the resin curing initiator is sensitive. The wavelength range of the emitted light falls typically within the blue part of the spectrum. It can be seen from FIG. 2 and 3 of the '696 patent that LEDs are typically arrayed in an arcuate configuration about a focal point 38. The apparatus is reported to be useful for curing dental resins including a 1:1 mixture by weight of bis-phenol-2bis (2-hydroxypropyl) methacrylate and tri (ethylene glycol) dimethacrylate monomers. The resinous mixture may further include a camphorquinone photoinitiator and a tertiary amine reducing agent. Fillers such as silica particles and colorants are typically included to achieve the desired hardness level and color.
U.S. Pat. No. 6,602,074 to Suh et al. discloses a process for preparing dental restorations which includes the application of light to a composite of intensity sufficient to penetrate the composite to initiate polymerization. Light application is suspended for a period of time sufficient to allow for the relaxation of internal stresses created by the initial polymerization of the composite. (Claim 1 requires suspension of the light for a period of 10 seconds.) Light is subsequently applied to the composite to complete polymerization.
WIPO Publication No. WO 99/35995 (essentially the same as U.S. Pat. No. 6,200,134) to Kovac et al. discloses a curing device for curing light sensitive compounds. The device includes generally a housing and an array of solid state light emitting diodes for having wavelengths in the range of 400-500 nm. Preferably, a peak wavelength of 470 nm is generated. The device further comprises an optical fiber light pipe for capturing the light and transmitting a beam of the light to the dental or other work surface containing a light curable compound. An optical lens may be used for focusing the light into the light pipe. It is noted on page 8 of this publication that 200-500 LEDs are used for creating the necessary light power needed for curing available dental compounds. In one embodiment of the device described, 96 LEDs are used whereas in a prototype e.g., an embodiment was made wherein 9 LEDs were utilized. See page 14. It is further noted in the publication that LEDs which include integral lenses may be employed. The discussion on page 20 and following notes that state that radiated power levels of approximately 200 mW/cm2 or greater are generally necessary for curing the available dental compounds. Other intensities may be necessary for curing other light sensitive compounds.
There is a need for methods of curing polymerizable compositions where the cured composition is of a higher quality. There is also a need to reduce the time necessary to cure polymerizable compositions. The applicants in the present invention have surprisingly found that, contrary to standard practices, the use of optical radiation with specific pulse duration and repetition rate yields better quality of cure than using comparable continuous wave optical radiation.